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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="metaparse.user_manual"></a><a name="manual"></a><a class="link" href="user_manual.html" title="User manual">User manual</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser">What is a parser</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.parsing_based_on_constexpr">Parsing
      based on <code class="computeroutput"><span class="keyword">constexpr</span></code></a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_types_of_grammars_can_be_us">What
      types of grammars can be used?</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="metaparse.user_manual.what_is_a_parser"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser" title="What is a parser">What is a parser</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.the_input_of_the_parsers">The
        input of the parsers</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.source_positions">Source
        positions</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.error_handling">Error
        handling</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.some_examples_of_simple_parsers">Some
        examples of simple parsers</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.combining_parsers">Combining
        parsers</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.sequence">Sequence</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition">Repetition</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.what_can_be_built_from_a_compile">What
        can be built from a compile-time string?</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.grammars">Grammars</a></span></dt>
</dl></div>
<p>
        See the <a class="link" href="reference.html#parser">parser</a> section of the <a class="link" href="reference.html#reference">reference</a>
        for the explanation of what a parser is.
      </p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.the_input_of_the_parsers"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.the_input_of_the_parsers" title="The input of the parsers">The
        input of the parsers</a>
</h4></div></div></div>
<p>
          Parsers take a <a class="link" href="reference.html#string"><code class="computeroutput"><span class="identifier">string</span></code></a>
          as input, which represents a string for template metaprograms. For example
          the string <code class="computeroutput"><span class="string">"Hello World!"</span></code>
          can be defined the following way:
        </p>
<pre class="programlisting"><span class="identifier">string</span><span class="special">&lt;</span><span class="char">'H'</span><span class="special">,</span><span class="char">'e'</span><span class="special">,</span><span class="char">'l'</span><span class="special">,</span><span class="char">'l'</span><span class="special">,</span><span class="char">'o'</span><span class="special">,</span><span class="char">' '</span><span class="special">,</span><span class="char">'W'</span><span class="special">,</span><span class="char">'o'</span><span class="special">,</span><span class="char">'r'</span><span class="special">,</span><span class="char">'l'</span><span class="special">,</span><span class="char">'d'</span><span class="special">,</span><span class="char">'!'</span><span class="special">&gt;</span>
</pre>
<p>
          This syntax makes the input of the parsers difficult to read. Metaparse
          works with compilers using C++98, but the input of the parsers has to be
          defined the way it is described above.
        </p>
<p>
          Based on <code class="computeroutput"><span class="keyword">constexpr</span></code>, a feature
          provided by C++11, Metaparse provides a macro, <a class="link" href="reference.html#BOOST_METAPARSE_STRING"><code class="computeroutput"><span class="identifier">BOOST_METAPARSE_STRING</span></code></a> for defining
          strings:
        </p>
<pre class="programlisting"><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"Hello World!"</span><span class="special">)</span>
</pre>
<p>
          This defines a <a class="link" href="reference.html#string"><code class="computeroutput"><span class="identifier">string</span></code></a>
          as well, however, it is easier to read. The maximum length of the string
          that can be defined this way is limited, however, this limit is configurable.
          It is specified by the <code class="computeroutput"><span class="identifier">BOOST_METAPARSE_LIMIT_STRING_SIZE</span></code>
          macro.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.source_positions"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.source_positions" title="Source positions">Source
        positions</a>
</h4></div></div></div>
<p>
          A source position is described using a compile-time data structure. The
          following functions can be used to query it:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              <a class="link" href="reference.html#get_col"><code class="computeroutput"><span class="identifier">get_col</span></code></a>
            </li>
<li class="listitem">
              <a class="link" href="reference.html#get_line"><code class="computeroutput"><span class="identifier">get_line</span></code></a>
            </li>
</ul></div>
<p>
          The beginning of the input is <a class="link" href="reference.html#start"><code class="computeroutput"><span class="identifier">start</span></code></a>
          which requires <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">metaparse</span><span class="special">/</span><span class="identifier">start</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code> to be included.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.error_handling"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.error_handling" title="Error handling">Error
        handling</a>
</h4></div></div></div>
<p>
          An error is described using a compile-time data structure. It contains
          information about the source position where the error was detected and
          some <a class="link" href="reference.html#parsing_error_message">description</a> about the
          error. <a class="link" href="reference.html#debug_parsing_error"><code class="computeroutput"><span class="identifier">debug_parsing_error</span></code></a>
          can be used to display the error message. Metaparse provides the <a class="link" href="reference.html#BOOST_METAPARSE_DEFINE_ERROR"><code class="computeroutput"><span class="identifier">BOOST_METAPARSE_DEFINE_ERROR</span></code></a>
          macro for defining simple <a class="link" href="reference.html#parsing_error_message">parsing
          error message</a>s.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.some_examples_of_simple_parsers"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.some_examples_of_simple_parsers" title="Some examples of simple parsers">Some
        examples of simple parsers</a>
</h4></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              A parser that parses nothing and always succeeds is <a class="link" href="reference.html#return_"><code class="computeroutput"><span class="identifier">return_</span></code></a>.
            </li>
<li class="listitem">
              A parser that always fails is <a class="link" href="reference.html#fail"><code class="computeroutput"><span class="identifier">fail</span></code></a>.
            </li>
<li class="listitem">
              A parser that parses one character and returns the parsed character
              as the result is <a class="link" href="reference.html#one_char"><code class="computeroutput"><span class="identifier">one_char</span></code></a>.
            </li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.combining_parsers"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.combining_parsers" title="Combining parsers">Combining
        parsers</a>
</h4></div></div></div>
<p>
          Complex parsers can be built by combining simple parsers. The parser library
          contains a number of parser combinators that build new parsers from already
          existing ones.
        </p>
<p>
          For example <a class="link" href="reference.html#accept_when"><code class="computeroutput"><span class="identifier">accept_when</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">Parser</span><span class="special">,</span> <span class="identifier">Predicate</span><span class="special">,</span> <span class="identifier">RejectErrorMsg</span><span class="special">&gt;</span></code> is a parser. It uses <code class="computeroutput"><span class="identifier">Parser</span></code> to parse the input. When <code class="computeroutput"><span class="identifier">Parser</span></code> rejects the input, the combinator
          returns the error <code class="computeroutput"><span class="identifier">Parser</span></code>
          failed with. When <code class="computeroutput"><span class="identifier">Parser</span></code>
          is successful, the combinator validates the result using <code class="computeroutput"><span class="identifier">Predicate</span></code>. If the predicate returns true,
          the combinator accepts the input, otherwise it generates an error with
          the message <code class="computeroutput"><span class="identifier">RejectErrorMsg</span></code>.
        </p>
<p>
          Having <a class="link" href="reference.html#accept_when"><code class="computeroutput"><span class="identifier">accept_when</span></code></a>,
          <a class="link" href="reference.html#one_char"><code class="computeroutput"><span class="identifier">one_char</span></code></a>
          can be used to build parsers that accept only digit characters, only whitespaces,
          etc. For example <a class="link" href="reference.html#digit"><code class="computeroutput"><span class="identifier">digit</span></code></a>
          accepts only digit characters:
        </p>
<pre class="programlisting"><span class="keyword">typedef</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">metaparse</span><span class="special">::</span><span class="identifier">accept_when</span><span class="special">&lt;</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">metaparse</span><span class="special">::</span><span class="identifier">one_char</span><span class="special">,</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">metaparse</span><span class="special">::</span><span class="identifier">util</span><span class="special">::</span><span class="identifier">is_digit</span><span class="special">,</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">metaparse</span><span class="special">::</span><span class="identifier">errors</span><span class="special">::</span><span class="identifier">digit_expected</span>
  <span class="special">&gt;</span>
  <span class="identifier">digit</span><span class="special">;</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.sequence"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.sequence" title="Sequence">Sequence</a>
</h4></div></div></div>
<p>
          The result of a successful parsing is some value and the remaining string
          that was not parsed. The remaining string can be processed by another parser.
          The parser library provides a parser combinator, <a class="link" href="reference.html#sequence"><code class="computeroutput"><span class="identifier">sequence</span></code></a>, that takes a number
          of parsers as arguments and builds a new parser from them that:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              Parses the input using the first parser
            </li>
<li class="listitem">
              If parsing succeeds, it parses the remaining string with the second
              parser
            </li>
<li class="listitem">
              It continues applying the parsers in order as long as they succeed
            </li>
<li class="listitem">
              If all of them succeed, it returns the list of results
            </li>
<li class="listitem">
              If any of the parsers fails, the combinator fails as well and returns
              the error the first failing parser returned with
            </li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition"></a><a name="repetition"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition" title="Repetition">Repetition</a>
</h4></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl">Introducing
          foldl</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldr">Introducing
          foldr</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl_start_with_par">Introducing
          foldl_start_with_parser</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldr_start_with_par">Introducing
          foldr_start_with_parser</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl_reject_incompl">Introducing
          foldl_reject_incomplete_start_with_parser</a></span></dt>
<dt><span class="section"><a href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.finding_the_right_folding_parser">Finding
          the right folding parser combinator</a></span></dt>
</dl></div>
<p>
          It is a common thing to parse a list of things of unknown length. As an
          example let's start with something simple: the text is a list of numbers.
          For example:
        </p>
<pre class="programlisting"><span class="number">11</span> <span class="number">13</span> <span class="number">3</span> <span class="number">21</span>
</pre>
<p>
          We want the result of parsing to be the sum of these values. Metaparse
          provides the <a class="link" href="reference.html#int_"><code class="computeroutput"><span class="identifier">int_</span></code></a>
          parser we can use to parse one of these numbers. Metaparse provides the
          <a class="link" href="reference.html#token"><code class="computeroutput"><span class="identifier">token</span></code></a>
          combinator to consume the whitespaces after the number. So the following
          parser parses one number and the whitespaces after it:
        </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">int_token</span> <span class="special">=</span> <span class="identifier">token</span><span class="special">&lt;</span><span class="identifier">int_</span><span class="special">&gt;;</span>
</pre>
<p>
          The result of parsing is a boxed integer value: the value of the parsed
          number. For example parsing <a class="link" href="reference.html#BOOST_METAPARSE_STRING"><code class="computeroutput"><span class="identifier">BOOST_METAPARSE_STRING</span></code></a><code class="computeroutput"><span class="special">(</span><span class="string">"13 "</span><span class="special">)</span></code> gives <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">13</span><span class="special">&gt;</span></code>
          as the result.
        </p>
<p>
          Our example input is a list of numbers. Each number can be parsed by <code class="computeroutput"><span class="identifier">int_token</span></code>:
        </p>
<p>
          <span class="inlinemediaobject"><img src="../images/metaparse/repeated_diag0.png" width="70%"></span>
        </p>
<p>
          This diagram shows how the repeated application of <code class="computeroutput"><span class="identifier">int_token</span></code>
          can parse the example input. Metaparse provides the <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a> parser to easily implement
          this. The result of parsing is a typelist: the list of the individual numbers.
        </p>
<p>
          <span class="inlinemediaobject"><img src="../images/metaparse/repeated_diag1.png" width="70%"></span>
        </p>
<p>
          This diagram shows how <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code> works. It uses the <code class="computeroutput"><span class="identifier">int_token</span></code>
          parser repeatedly and builds a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span></code>
          from the results it provides.
        </p>
<p>
          But we need the sum of these, so we need to summarise the result. We can
          do this by wrapping our parser, <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code>
          with <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a>.
          That gives us the opportunity to specify a function transforming this typelist
          to some other value - the sum of the elements in our case. Initially let's
          ignore how to summarise the elements in the vector. Let's assume that it
          can be implemented by a lambda expression and use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">&lt;...&gt;::</span><span class="identifier">type</span></code>
          representing that lambda expression. Here is an example using <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> and this lambda expression:
        </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">sum_parser</span> <span class="special">=</span>
  <span class="identifier">transform</span><span class="special">&lt;</span>
    <span class="identifier">repeated</span><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;,</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">&lt;...&gt;::</span><span class="identifier">type</span>
  <span class="special">&gt;;</span>
</pre>
<p>
          The <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a><code class="computeroutput"><span class="special">&lt;&gt;</span></code> parser combinator wraps the <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code> to build the parser we need. Here is
          a diagram showing how it works:
        </p>
<p>
          <span class="inlinemediaobject"><img src="../images/metaparse/repeated_diag2.png" width="70%"></span>
        </p>
<p>
          As the diagram shows, the <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a><code class="computeroutput"><span class="special">&lt;</span></code><a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;,</span>
          <span class="special">...&gt;</span></code> parser parses the input
          using <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code> and then does some processing on the
          result of parsing.
        </p>
<p>
          Let's implement the missing lambda expression that tells <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> how to change the result
          coming from <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code>. We can summarise the numbers in a
          typelist by using Boost.MPL's <code class="computeroutput"><span class="identifier">fold</span></code>
          or <code class="computeroutput"><span class="identifier">accumulate</span></code>. Here is
          an example doing that:
        </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">sum_op</span> <span class="special">=</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">_1</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">_2</span><span class="special">&gt;&gt;::</span><span class="identifier">type</span><span class="special">;</span>

<span class="keyword">using</span> <span class="identifier">sum_parser</span> <span class="special">=</span>
  <span class="identifier">transform</span><span class="special">&lt;</span>
    <span class="identifier">repeated</span><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;,</span>
    <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">&lt;</span>
      <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">_1</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;,</span> <span class="identifier">sum_op</span><span class="special">&gt;</span>
    <span class="special">&gt;::</span><span class="identifier">type</span>
  <span class="special">&gt;;</span>
</pre>
<p>
          Here is an extended version of the above diagram showing what happens here:
        </p>
<p>
          <span class="inlinemediaobject"><img src="../images/metaparse/repeated_diag3.png" width="70%"></span>
        </p>
<p>
          This example parses the input, builds the list of numbers and then loops
          over it and summarises the values. It starts with the second argument of
          <code class="computeroutput"><span class="identifier">fold</span></code>, <code class="computeroutput"><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>
          and adds every item of the list of numbers (which is the result of the
          parser <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code>) one by one.
        </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
            Note that <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a>
            wraps another parser, <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">&gt;</span></code> here. It parses the input with that
            parser, gets the result of that parsing and changes that result. <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a>
            itself will be a parser returning that updated result.
          </p></td></tr>
</table></div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl"></a><a name="introducing-foldl"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl" title="Introducing foldl">Introducing
          foldl</a>
</h5></div></div></div>
<p>
            It works, however, this is rather inefficient: it has a loop parsing
            the integers one by one, building a typelist and then it loops over this
            typelist to summarise the result. Using template metaprograms in your
            applications can have a serious impact on the compiler's memory usage
            and the speed of the compilation, therefore I recommend being careful
            with these things.
          </p>
<p>
            Metaparse offers more efficient ways of achieving the same result. You
            don't need two loops: you can merge them together and add every number
            to your summary right after parsing it. Metaparse offers the <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a> for this.
          </p>
<p>
            With <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>
            you specify:
          </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
                the parser to parse the individual elements of the list (which is
                <code class="computeroutput"><span class="identifier">int_token</span></code> in our
                example)
              </li>
<li class="listitem">
                the initial value used for folding (which is <code class="computeroutput"><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code> in our example)
              </li>
<li class="listitem">
                the forward operation merging the sub-result we have so far and the
                value coming from the last application of the parser (this was <code class="computeroutput"><span class="identifier">sum_op</span></code> in our example)
              </li>
</ul></div>
<p>
            Our parser can be implemented this way:
          </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">better_sum_parser</span> <span class="special">=</span> <span class="identifier">foldl</span><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;,</span> <span class="identifier">sum_op</span><span class="special">&gt;;</span>
</pre>
<p>
            As you can see the implementation of the parser is more compact. Here
            is a diagram showing what happens when you use this parser to parse some
            input:
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/foldl_diag1.png" width="70%"></span>
          </p>
<p>
            As you can see, not only the implementation of the parser is more compact,
            but it achieves the same result by doing less as well. It parses the
            input by applying <code class="computeroutput"><span class="identifier">int_token</span></code>
            repeatedly, just like the previous solution. But it produces the final
            result without building a typelist as an internal step. Here is how it
            works internally:
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/foldl_diag2.png" width="70%"></span>
          </p>
<p>
            It summarises the results of the repeated <code class="computeroutput"><span class="identifier">int_token</span></code>
            application using <code class="computeroutput"><span class="identifier">sum_op</span></code>.
            This implementation is more efficient. It accepts an empty string as
            a valid input: the sum of it is <code class="computeroutput"><span class="number">0</span></code>.
            It may be good for you, in which case you are done. If you don't wan
            to accept it, you can use <a class="link" href="reference.html#foldl1"><code class="computeroutput"><span class="identifier">foldl1</span></code></a>
            instead of <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>.
            This is the same, but it rejects empty input. (Metaparse offers <a class="link" href="reference.html#repeated1"><code class="computeroutput"><span class="identifier">repeated1</span></code></a>
            as well if you choose the first approach and would like to reject empty
            string)
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition.introducing_foldr"></a><a name="introducing-foldr"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldr" title="Introducing foldr">Introducing
          foldr</a>
</h5></div></div></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Note that if you are reading this manual for the first time, you probably
              want to skip this section and proceed with <a class="link" href="user_manual.html#introducing-foldl_start_with_parser">Introducing
              foldl_start_with_parser</a>
            </p></td></tr>
</table></div>
<p>
            You might have noticed that Metaparse offers <a class="link" href="reference.html#foldr"><code class="computeroutput"><span class="identifier">foldr</span></code></a> as well. The difference
            between <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>
            and <a class="link" href="reference.html#foldr"><code class="computeroutput"><span class="identifier">foldr</span></code></a>
            is the direction in which the results are summarised. (<code class="computeroutput"><span class="identifier">l</span></code> stands for <span class="emphasis"><em>from the Left</em></span>
            and <code class="computeroutput"><span class="identifier">r</span></code> stands for <span class="emphasis"><em>from
            the Right</em></span>) Here is a diagram showing how <code class="computeroutput"><span class="identifier">better_sum_parser</span></code>
            works if it is implemented using <a class="link" href="reference.html#foldr"><code class="computeroutput"><span class="identifier">foldr</span></code></a>:
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/foldr_diag1.png" width="70%"></span>
          </p>
<p>
            As you can see this is very similar to using <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>, but the results coming
            out of the individual applications of <code class="computeroutput"><span class="identifier">int_token</span></code>
            are summarised in a right-to-left order. As <code class="computeroutput"><span class="identifier">sum_op</span></code>
            is addition, it does not affect the end result, but in other cases it
            might.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Note that the implementation of <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a> is more efficient than
              <a class="link" href="reference.html#foldr"><code class="computeroutput"><span class="identifier">foldr</span></code></a>.
              Prefer <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>
              whenever possible.
            </p></td></tr>
</table></div>
<p>
            As you might expect it, Metaparse offers <a class="link" href="reference.html#foldr1"><code class="computeroutput"><span class="identifier">foldr1</span></code></a> as well, which folds
            from the right and rejects empty input.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl_start_with_par"></a><a name="introducing-foldl_start_with_parser"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl_start_with_par" title="Introducing foldl_start_with_parser">Introducing
          foldl_start_with_parser</a>
</h5></div></div></div>
<p>
            Let's change the grammar of our little language. Instead of a list of
            numbers, let's expect numbers separated by a <code class="computeroutput"><span class="special">+</span></code>
            symbol. Our example input becomes the following:
          </p>
<pre class="programlisting"><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"11 + 13 + 3 + 21"</span><span class="special">)</span>
</pre>
<p>
            Parsing it with <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>
            or <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a>
            is difficult: there has to be a <code class="computeroutput"><span class="special">+</span></code>
            symbol before every element <span class="emphasis"><em>except</em></span> the first one.
            None of the already introduced repetition constructs offer a way of treating
            the first element in a different way.
          </p>
<p>
            If we forget about the first number for a moment, the rest of the input
            is <code class="computeroutput"><span class="string">"+ 13 + 3 + 21"</span></code>.
            This can easily be parsed by <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>
            (or <a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a>):
          </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">plus_token</span> <span class="special">=</span> <span class="identifier">token</span><span class="special">&lt;</span><span class="identifier">lit_c</span><span class="special">&lt;</span><span class="char">'+'</span><span class="special">&gt;&gt;;</span>
<span class="keyword">using</span> <span class="identifier">plus_int</span> <span class="special">=</span> <span class="identifier">last_of</span><span class="special">&lt;</span><span class="identifier">plus_token</span><span class="special">,</span> <span class="identifier">int_token</span><span class="special">&gt;;</span>

<span class="keyword">using</span> <span class="identifier">sum_parser2</span> <span class="special">=</span> <span class="identifier">foldl</span><span class="special">&lt;</span><span class="identifier">plus_int</span><span class="special">,</span> <span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;,</span> <span class="identifier">sum_op</span><span class="special">&gt;;</span>
</pre>
<p>
            It uses <code class="computeroutput"><span class="identifier">plus_int</span></code>, that
            is <a class="link" href="reference.html#last_of"><code class="computeroutput"><span class="identifier">last_of</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">plus_token</span><span class="special">,</span> <span class="identifier">int_token</span><span class="special">&gt;</span></code> as the parser that is used repeatedly
            to get the numbers. It does the following:
          </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
                Uses <code class="computeroutput"><span class="identifier">plus_token</span></code> to
                parse the <code class="computeroutput"><span class="special">+</span></code> symbol and
                any whitespace that might follow it.
              </li>
<li class="listitem">
                Uses then <code class="computeroutput"><span class="identifier">int_token</span></code>
                to parse the number
              </li>
<li class="listitem">
                Combines the above two with <a class="link" href="reference.html#last_of"><code class="computeroutput"><span class="identifier">last_of</span></code></a> to use both parsers
                in order and keep only the result of using the second one (the result
                of parsing the <code class="computeroutput"><span class="special">+</span></code> symbol
                is thrown away - we don't care about it).
              </li>
</ul></div>
<p>
            This way <a class="link" href="reference.html#last_of"><code class="computeroutput"><span class="identifier">last_of</span></code></a><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">plus_token</span><span class="special">,</span> <span class="identifier">int_token</span><span class="special">&gt;</span></code> returns the value of the number as
            the result of parsing, just like our previous parser, <code class="computeroutput"><span class="identifier">int_token</span></code>
            did. Because of this, it can be used as a drop-in replacement of <code class="computeroutput"><span class="identifier">int_token</span></code> in the previous example and
            we get a parser for our updated language. Or at least for all number
            except the first one.
          </p>
<p>
            This <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>
            can not parse the first element, because it expects a <code class="computeroutput"><span class="special">+</span></code>
            symbol before every number. You might think of making the <code class="computeroutput"><span class="special">+</span></code> symbol optional in the above approach
            - don't do that. It makes the parser accept <code class="computeroutput"><span class="string">"11
            + 13 3 21"</span></code> as well as the <code class="computeroutput"><span class="special">+</span></code>
            symbol is now optional <span class="emphasis"><em>everywhere</em></span>.
          </p>
<p>
            What you could do is parsing the first element with <code class="computeroutput"><span class="identifier">int_token</span></code>,
            the rest of the elements with the above <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>-based solution and add
            the result of the two. This is left as an exercise to the reader.
          </p>
<p>
            Metaparse offers <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a> to implement
            this. <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
            is the same as <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a>.
            The difference is that instead of an initial value to combine the list
            elements with it takes an <span class="emphasis"><em>initial parser</em></span>:
          </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">plus_token</span> <span class="special">=</span> <span class="identifier">token</span><span class="special">&lt;</span><span class="identifier">lit_c</span><span class="special">&lt;</span><span class="char">'+'</span><span class="special">&gt;&gt;;</span>
<span class="keyword">using</span> <span class="identifier">plus_int</span> <span class="special">=</span> <span class="identifier">last_of</span><span class="special">&lt;</span><span class="identifier">plus_token</span><span class="special">,</span> <span class="identifier">int_token</span><span class="special">&gt;;</span>

<span class="keyword">using</span> <span class="identifier">sum_parser3</span> <span class="special">=</span> <span class="identifier">foldl_start_with_parser</span><span class="special">&lt;</span><span class="identifier">plus_int</span><span class="special">,</span> <span class="identifier">int_token</span><span class="special">,</span> <span class="identifier">sum_op</span><span class="special">&gt;;</span>
</pre>
<p>
            <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
            starts with applying that initial parser and uses the result it returns
            as the initial value for folding. It does the same as <a class="link" href="reference.html#foldl"><code class="computeroutput"><span class="identifier">foldl</span></code></a> after that. The following
            diagram shows how it can be used to parse a list of numbers separated
            by <code class="computeroutput"><span class="special">+</span></code> symbols:
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/foldl_start_with_parser_diag1.png" width="70%"></span>
          </p>
<p>
            As the diagram shows, it start parsing the list of numbers with <code class="computeroutput"><span class="identifier">int_token</span></code>, uses its value as the starting
            value for folding (earlier approaches were using the value <code class="computeroutput"><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code> as
            this starting value). Then it parses all elements of the list by using
            <code class="computeroutput"><span class="identifier">plus_int</span></code> multiple times.
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition.introducing_foldr_start_with_par"></a><a name="introducing-foldr_start_with_parser"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldr_start_with_par" title="Introducing foldr_start_with_parser">Introducing
          foldr_start_with_parser</a>
</h5></div></div></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Note that if you are reading this manual for the first time, you probably
              want to skip this section and try creating some parsers using <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
              instead.
            </p></td></tr>
</table></div>
<p>
            <a href="../foldl_start_with_parser.hpp" target="_top"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
            has its <span class="emphasis"><em>from the right</em></span> pair, <a class="link" href="reference.html#foldr_start_with_parser"><code class="computeroutput"><span class="identifier">foldr_start_with_parser</span></code></a>. It
            uses the same elements as <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a> but
            in a different order. Here is a parser for our example language implemented
            with <a class="link" href="reference.html#foldr_start_with_parser"><code class="computeroutput"><span class="identifier">foldr_start_with_parser</span></code></a>:
          </p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">plus_token</span> <span class="special">=</span> <span class="identifier">token</span><span class="special">&lt;</span><span class="identifier">lit_c</span><span class="special">&lt;</span><span class="char">'+'</span><span class="special">&gt;&gt;;</span>
<span class="keyword">using</span> <span class="identifier">int_plus</span> <span class="special">=</span> <span class="identifier">first_of</span><span class="special">&lt;</span><span class="identifier">int_token</span><span class="special">,</span> <span class="identifier">plus_token</span><span class="special">&gt;;</span>

<span class="keyword">using</span> <span class="identifier">sum_parser4</span> <span class="special">=</span> <span class="identifier">foldr_start_with_parser</span><span class="special">&lt;</span><span class="identifier">int_plus</span><span class="special">,</span> <span class="identifier">int_token</span><span class="special">,</span> <span class="identifier">sum_op</span><span class="special">&gt;;</span>
</pre>
<p>
            Note that it uses <code class="computeroutput"><span class="identifier">int_plus</span></code>
            instead of <code class="computeroutput"><span class="identifier">plus_int</span></code>.
            This is because the parser the initial value for folding comes from is
            used after <code class="computeroutput"><span class="identifier">int_plus</span></code> has
            parsed the input as many times as it could. It might sound strange for
            the first time, but the following diagram should help you understand
            how it works:
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/foldr_start_with_parser_diag1.png" width="70%"></span>
          </p>
<p>
            As you can see, it starts with the parser that is applied repeatedly
            on the input, thus instead of parsing <code class="computeroutput"><span class="identifier">plus_token</span>
            <span class="identifier">int_token</span></code> repeatedly, we need
            to parse <code class="computeroutput"><span class="identifier">int_token</span> <span class="identifier">plus_token</span></code>
            repeatedly. The last number is not followed by <code class="computeroutput"><span class="special">+</span></code>,
            thus <code class="computeroutput"><span class="identifier">int_plus</span></code> fails to
            parse it and it stops the iteration. <a class="link" href="reference.html#foldr_start_with_parser"><code class="computeroutput"><span class="identifier">foldr_start_with_parser</span></code></a> then
            uses the other parser, <code class="computeroutput"><span class="identifier">int_token</span></code>
            to parse the input. It succeeds and the result it returns is used as
            the starting value for folding from the right.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Note that as the above description also suggests, the implementation
              of <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
              is more efficient than <a class="link" href="reference.html#foldr_start_with_parser"><code class="computeroutput"><span class="identifier">foldr_start_with_parser</span></code></a>. Prefer
              <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
              whenever possible.
            </p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl_reject_incompl"></a><a name="introducing-foldl_reject_incomplete_start_with_parser"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.introducing_foldl_reject_incompl" title="Introducing foldl_reject_incomplete_start_with_parser">Introducing
          foldl_reject_incomplete_start_with_parser</a>
</h5></div></div></div>
<p>
            Using a parser built with <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a> we can
            parse the input when the input is correct. However, it is not always
            the case. Consider the following input for example:
          </p>
<pre class="programlisting"><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"11 + 13 + 3 + 21 +"</span><span class="special">)</span>
</pre>
<p>
            This is an invalid expression. However, if we parse it using the <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>-based
            parser presented earlier (<code class="computeroutput"><span class="identifier">sum_parser3</span></code>),
            it accepts the input and the result is <code class="computeroutput"><span class="number">48</span></code>.
            This is because <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a> parses
            the input <span class="emphasis"><em>as long as it can</em></span>. It parses the first<code class="computeroutput"><span class="identifier">int_token</span></code> (<code class="computeroutput"><span class="number">11</span></code>)
            and then it starts parsing the <code class="computeroutput"><span class="identifier">plus_int</span></code>
            elements (<code class="computeroutput"><span class="special">+</span> <span class="number">13</span></code>,
            <code class="computeroutput"><span class="special">+</span> <span class="number">3</span></code>,
            <code class="computeroutput"><span class="special">+</span> <span class="number">21</span></code>).
            After parsing all of these, it tries to parse the remaining <code class="computeroutput"><span class="string">" +"</span></code> input using <code class="computeroutput"><span class="identifier">plus_int</span></code> which fails and therefore
            <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>
            stops after <code class="computeroutput"><span class="special">+</span> <span class="number">21</span></code>.
          </p>
<p>
            The problem is that the parser parses the longest sub-expression starting
            from the beginning, that represents a valid expression. The rest is ignored.
            The parser can be wrapped by <a class="link" href="reference.html#entire_input"><code class="computeroutput"><span class="identifier">entire_input</span></code></a> to make sure to
            reject expressions with invalid extra characters at the end, however,
            that won't make the error message useful. (<a class="link" href="reference.html#entire_input"><code class="computeroutput"><span class="identifier">entire_input</span></code></a> can only tell the
            author of the invalid expression that after <code class="computeroutput"><span class="special">+</span>
            <span class="number">21</span></code> is something wrong).
          </p>
<p>
            Metaparse provides <a class="link" href="reference.html#foldl_reject_incomplete_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_reject_incomplete_start_with_parser</span></code></a>,
            which does the same as <a class="link" href="reference.html#foldl_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_start_with_parser</span></code></a>, except
            that once no further repetitions are found, it checks <span class="emphasis"><em>where</em></span>
            the repeated parser (in our example <code class="computeroutput"><span class="identifier">plus_int</span></code>)
            fails. When it can make any progress (eg. it finds a <code class="computeroutput"><span class="special">+</span></code>
            symbol), then <a class="link" href="reference.html#foldl_reject_incomplete_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_reject_incomplete_start_with_parser</span></code></a>
            assumes, that the expression's author intended to make the repetition
            longer, but made a mistake and propagates the error message coming from
            that last broken expression.
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/foldl_reject_incomplete_start_with_parser_diag1.png" width="70%"></span>
          </p>
<p>
            The above diagram shows how <a class="link" href="reference.html#foldl_reject_incomplete_start_with_parser"><code class="computeroutput"><span class="identifier">foldl_reject_incomplete_start_with_parser</span></code></a>
            parses the example invalid input and how it fails. This can be used for
            better error reporting from the parsers.
          </p>
<p>
            Other folding parsers also have their <code class="computeroutput"><span class="identifier">f</span></code>
            version. (eg. <a class="link" href="reference.html#foldr_reject_incomplete"><code class="computeroutput"><span class="identifier">foldr_reject_incomplete</span></code></a>,
            <a class="link" href="reference.html#foldl_reject_incomplete1"><code class="computeroutput"><span class="identifier">foldl_reject_incomplete1</span></code></a>,
            etc).
          </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="metaparse.user_manual.what_is_a_parser.repetition.finding_the_right_folding_parser"></a><a name="finding-the-right-folding-parser-combinator"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.repetition.finding_the_right_folding_parser" title="Finding the right folding parser combinator">Finding
          the right folding parser combinator</a>
</h5></div></div></div>
<p>
            As you might have noticed, there are a lot of different folding parser
            combinators. To help you find the right one, the following naming convention
            is used:
          </p>
<p>
            <span class="inlinemediaobject"><img src="../images/metaparse/folds.png" width="70%"></span>
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Note that there is no <code class="computeroutput"><span class="identifier">foldr_reject_incomplete_start_with_parser</span></code>.
              The <code class="computeroutput"><span class="identifier">p</span></code> version of the
              right-folding parsers applies the special parser, whose result is the
              initial value, after the repeated elements. Therefore, when the parser
              parsing one repeated element fails, <code class="computeroutput"><span class="identifier">foldr_start_with_parser</span></code>
              would apply that special final parser instead of checking how the repeated
              element's parser failed.
            </p></td></tr>
</table></div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.what_can_be_built_from_a_compile"></a><a name="result_types"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.what_can_be_built_from_a_compile" title="What can be built from a compile-time string?">What
        can be built from a compile-time string?</a>
</h4></div></div></div>
<p>
          Parsers built using Metaparse are template metaprograms parsing text (or
          code) at compile-time. Here is a list of things that can be the "result"
          of parsing:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              A <span class="emphasis"><em>type</em></span>. An example for this is a parser parsing
              a <code class="computeroutput"><span class="identifier">printf</span></code> format string
              and returning the typelist (eg. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span></code>)
              of the expected arguments.
            </li>
<li class="listitem">
              A <span class="emphasis"><em>constant value</em></span>. An example for this is the result
              of a calculator language. See the <a class="link" href="getting_started_with_boost_metap.html#getting_started">Getting
              Started</a> section for further details.
            </li>
<li class="listitem">
              A <span class="emphasis"><em>runtime object</em></span>. A static runtime object can
              be generated that might be used at runtime. An example for this is
              parsing regular expressions at compile-time and building <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">xpressive</span><span class="special">::</span><span class="identifier">sregex</span></code> objects. See the <code class="computeroutput"><span class="identifier">regex</span></code> example of Metaparse for an
              example.
            </li>
<li class="listitem">
              A C++ <span class="emphasis"><em>function</em></span>, which might be called at runtime.
              A C++ function can be generated that can be called at runtime. It is
              good for generating native (and optimised) code from EDSLs. See the
              <code class="computeroutput"><span class="identifier">compile_to_native_code</span></code>
              example of Metaparse as an example for this.
            </li>
<li class="listitem">
              A <a class="link" href="reference.html#metafunction_class"><span class="emphasis"><em>template metafunction
              class</em></span></a>. The result of parsing might be a type, which
              is a <a class="link" href="reference.html#metafunction_class">template metafunction class</a>.
              This is good for building an EDSL for template metaprogramming. See
              the <code class="computeroutput"><span class="identifier">meta_hs</span></code> example
              of Metaparse as an example for this.
            </li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="metaparse.user_manual.what_is_a_parser.grammars"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_is_a_parser.grammars" title="Grammars">Grammars</a>
</h4></div></div></div>
<p>
          Metaparse provides a way to define grammars in a syntax that resembles
          EBNF. The <a class="link" href="reference.html#grammar"><code class="computeroutput"><span class="identifier">grammar</span></code></a>
          template can be used to define a grammar. It can be used the following
          way:
        </p>
<pre class="programlisting"><span class="identifier">grammar</span><span class="special">&lt;</span><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"plus_exp"</span><span class="special">)&gt;</span>
  <span class="special">::</span><span class="identifier">import</span><span class="special">&lt;</span><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"int_token"</span><span class="special">),</span> <span class="identifier">token</span><span class="special">&lt;</span><span class="identifier">int_</span><span class="special">&gt;&gt;::</span><span class="identifier">type</span>

  <span class="special">::</span><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"ws ::= (' ' | '\n' | '\r' | '\t')*"</span><span class="special">)&gt;::</span><span class="identifier">type</span>
  <span class="special">::</span><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"plus_token ::= '+' ws"</span><span class="special">),</span> <span class="identifier">front</span><span class="special">&lt;</span><span class="identifier">_1</span><span class="special">&gt;&gt;::</span><span class="identifier">type</span>
  <span class="special">::</span><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="string">"plus_exp ::= int_token (plus_token int_token)*"</span><span class="special">),</span> <span class="identifier">plus_action</span><span class="special">&gt;::</span><span class="identifier">type</span>
</pre>
<p>
          The code above defines a parser from a grammar definition. The start symbol
          of the grammar is <code class="computeroutput"><span class="identifier">plus_exp</span></code>.
          The lines beginning with <code class="computeroutput"><span class="special">::</span><span class="identifier">rule</span></code> define rules. Rules optionally have
          a semantic action, which is a metafunction class that transforms the result
          of parsing after the rule has been applied. Existing parsers can be bound
          to names and be used in the rules by importing them. Lines beginning with
          <code class="computeroutput"><span class="special">::</span><span class="identifier">import</span></code>
          bind existing parsers to names.
        </p>
<p>
          The result of a grammar definition is a parser which can be given to other
          parser combinators or be used directly. Given that grammars can import
          existing parsers and build new ones, they are parser combinators as well.
        </p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="metaparse.user_manual.parsing_based_on_constexpr"></a><a class="link" href="user_manual.html#metaparse.user_manual.parsing_based_on_constexpr" title="Parsing based on constexpr">Parsing
      based on <code class="computeroutput"><span class="keyword">constexpr</span></code></a>
</h3></div></div></div>
<p>
        Metaparse is based on template metaprogramming, however, C++11 provides
        <code class="computeroutput"><span class="keyword">constexpr</span></code>, which can be used
        for parsing at compile-time as well. While implementing parsers based on
        <code class="computeroutput"><span class="keyword">constexpr</span></code> is easier for a C++
        developer, since its syntax resembles the regular syntax of the language,
        the result of parsing has to be a <code class="computeroutput"><span class="keyword">constexpr</span></code>
        value. Parsers based on template metaprogramming can build types as the result
        of parsing. These types may be boxed <code class="computeroutput"><span class="keyword">constexpr</span></code>
        values but can be metafunction classes, classes with static functions which
        can be called at runtime, etc.
      </p>
<p>
        When a parser built with Metaparse needs a sub-parser for processing a part
        of the input text and generating a <code class="computeroutput"><span class="keyword">constexpr</span></code>
        value as the result of parsing, one can implement the sub-parser based on
        <code class="computeroutput"><span class="keyword">constexpr</span></code> functions. Metaparse
        can be integrated with them and lift their results into C++ template metaprogramming.
        An example demonstrating this feature can be found among the examples (<code class="computeroutput"><span class="identifier">constexpr_parser</span></code>). This capability makes
        it possible to integrate Metaparse with parsing libraries based on <code class="computeroutput"><span class="keyword">constexpr</span></code>.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="metaparse.user_manual.what_types_of_grammars_can_be_us"></a><a class="link" href="user_manual.html#metaparse.user_manual.what_types_of_grammars_can_be_us" title="What types of grammars can be used?">What
      types of grammars can be used?</a>
</h3></div></div></div>
<p>
        It is possible to write parsers for <span class="emphasis"><em>context free grammars</em></span>
        using Metaparse. However, this is not the most general category of grammars
        that can be used. As Metaparse is a highly extendable framework, it is not
        clear what should be considered to be the limit of Metaparse itself. For
        example Metaparse provides the <a class="link" href="reference.html#accept_when"><code class="computeroutput"><span class="identifier">accept_when</span></code></a> <a class="link" href="reference.html#parser_combinator">parser
        combinator</a>. It can be used to provide arbitrary predicates for enabled/disabling
        a specific rule. One can go as far as providing the Turing machine (as a
        <a class="link" href="reference.html#metafunction">metafunction</a>) of the entire grammar as
        a predicate, so one can build parsers for <span class="emphasis"><em>unrestricted grammars</em></span>
        that can be parsed using a Turing machine. Note that such a parser would
        not be considered to be a parser built with Metaparse, however, it is not
        clear how far a solution might go and still be considered using Metaparse.
      </p>
<p>
        Metaparse assumes that the parsers are <span class="emphasis"><em>deterministic</em></span>,
        as they have only "one" result. It is of course possible to write
        parsers and combinators that return a set (or list or some other container)
        of results as that "one" result, but that can be considered building
        a new parser library. There is no clear boundary for Metaparse.
      </p>
<p>
        Metaparse supports building <span class="emphasis"><em>top-down parsers</em></span> and <span class="emphasis"><em>left-recursion</em></span>
        is not supported as it would lead to infinite recursion. <span class="emphasis"><em>Right-recursion</em></span>
        is supported, however, in most cases the <a class="link" href="user_manual.html#repetition">iterative
        parser combinators</a> provide better alternatives.
      </p>
</div>
</div>
<div class="copyright-footer">Copyright © 2015 Abel Sinkovics<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div>
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